LAS VEGAS — Although Brock Lesnar left behind pro wrestling three years ago to become the UFC’s heavyweight champion, he still hasn’t got all the showtime out of his system.

Lesnar stopped Frank Mir with a merciless flurry of punches Saturday night, defending his title in the final bout of UFC 100 on a landmark weekend recognizing the best in his proliferating sport.

Lesnar then taunted his fallen opponent, made a two-handed obscene gesture at the booing crowd and insulted a UFC sponsor, putting an altogether unflattering end on what the league planned as a sparkling celebration of mixed martial arts.

The former WWE star just couldn’t resist being a heel, even though he apologized profusely afterward.

“I acted very unprofessionally after the fight,” Lesnar said of his second-round victory. “There was a lot of emotion in this bout, (but) I love this company, and I love to fight. I went into the entertainment business for a little while, and I guess there’s still a little bit of it in me.”

There were no such theatrics from Georges St. Pierre, who defended his welterweight title with exponentially more class in an unanimous decision over Thiago Alves in front of the sellout crowd at the Mandalay Bay Events Center. Dan Henderson also knocked out Michael Bisping with a single punch in a middleweight upset as the UFC rolled its odometer into triple digits.

But Lesnar (4-1) stole the show with what happened after his relentless series of right hands into the face of Mir, who handed Lesnar his only loss nearly 18 months ago.

“I was absolutely blown away,” said UFC president Dana White, who has labored to put a professional face on his sport. “In the history of the UFC, we’ve never had somebody do anything like that. … That’s not who he is. I’ve spent time with this guy. He gets in there, and the adrenaline starts kicking. The mouth is moving, but the brain isn’t sending stuff to the mouth.

“He’s a smart guy. He gets it, but I believe that what he said tonight, he’s been in the WWE for so long, he thinks he has to jump in there and be the heel or whatever.”

Lesnar, who turns 32 on Sunday, again moved upward in the eclectic trajectory of a former college wrestler who performed in the WWE and briefly suited up for the Minnesota Vikings before embarking on an MMA career just over three years ago.

Mir (12-4) beat Lesnar with a knee bar at UFC 81, forcing Lesnar to tap out 90 seconds into his second UFC fight. Lesnar, whose name recognition and menacing approach earned him lightning-quick shots at the sport’s biggest prizes, recovered by beating Heath Herring and then stopping Randy Couture last November to claim the heavyweight belt.

Lesnar used his 265 pounds to take position on top of Mir early, and Lesnar stayed on top throughout a dull first round, throwing punches that mostly did little. Mir made a bit of headway in the second, but was trapped against the octagon by Lesnar, who kept throwing right hands until Mir’s defense wilted.

Lesnar then pointed and taunted when Mir stood up, his face still caked in blood. After acknowledging the fans’ boos with two middle fingers, Lesnar took the microphone and looked pointedly at the Bud Light logo on the cage floor while announcing he would be drinking Coors Light to celebrate, “because Bud Light won’t pay me.”

Nearly 16 years after MMA pioneer Royce Gracie won three fights in one night to triumph at UFC 1 in Denver, the sport reached a centennial of sorts before a frenzied sellout crowd in the UFC’s hometown. The arena was packed well before the main bouts, including some fans who apparently paid more than $40,000 online for resold tickets.

The league has grown from a bit player in a fringe pastime to an estimated $1 billion company, and the weekend’s festivities reflected its success despite the widespread skepticism it still faces from other sports fans and some states’ lawmakers.

The 100th showcase was broadcast live in 75 countries, and thousands of fans without tickets for the event packed into closed-circuit shows all along the Strip. Tens of thousands attended the first UFC Fan Expo, immersing themselves in the lifestyle built around the sport.

St. Pierre (19-2), the classy Canadian star considered among the UFC’s pound-for-pound best, won every round on every scorecard while steadily punishing his Brazilian opponent with punches, kicks and repeated takedowns in his third title defense.

St. Pierre also injured his groin in the third, and Alves (22-5) managed to gain position for the only time in the fight. St. Pierre escaped and nearly finished it with a rear naked choke on Alves, who lost for the first time in eight fights.

St. Pierre blamed the injury on an abbreviated warmup period.

“I’m in really bad pain,” St. Pierre said. “I don’t know how long it’s going to take me (to recover). It’s not the first time it happened.”

In the undercard showdown between the veteran fighters who served as coaches on the past season of the UFC’s popular television show, Henderson (25-7) flattened Bisping with a spectacular right hand in the second round.